According to the prior art, commercial agricultural vehicles comprise a hydrostatic-mechanical powerdistribution transmission consisting of a continuously variable hydrostatic transmission with a unit having a variable volume, a constant volume, a summarizing transmission and a range-change transmission with several shiftable gears. The rotational speeds and torques of the hydrostatic transmission and the prime mover are synchronized by the summarizing transmission. By coupling the output shaft of the summarizing transmission with the output shaft, via different spur gear sets with the aid of gear clutches, different gears can be selected. Compared to mechanical transmissions, continuously variable transmissions have the advantage that in every driving mode it is possible to drive, at optimal motor rotational speeds, with no tractional interruptions occurring when shifting.
The hydrostatic transmissions in hydrostatic-mechanical power distribution transmissions have, as a rule, high-pressure limiting valves which limit the maximum working pressure of the hydrostatic transmission. The hydrostatic transmission is thereby protected from overstressing caused by too high of a working pressure.
Limitation of the high pressure also limits the torque over the hydrostatic branch of the power distribution transmission and thus also the traction of the vehicle. While the motor is running, opening of the valves for discharging the high pressure is felt in the vehicle by stopping the vehicle. In order to prevent unexpected stopping of the vehicle by the driver, in many applications this driving mode is asymptotically approximated by an active delay of the vehicle, thus communicating to the driver that the vehicle has reached its traction limit.
From the prior art it is known to mathematically determine the hydrostatic pressure via the ratio of the real and the theoretical rotational speed ratio (that is, from the hydrostatic ratio). For this purpose, characteristic fields of the hydrostatic leakage, depending on working pressure and other physical parameters is used. Due to manufacturing tolerances and wear resulting from operation, even despite calibrations the mathematical results disadvantageously diverge from the positively existing working pressure.
In agricultural and municipal uses, there are cases in which a very strong torque might be needed for a few seconds even though it is not required to drive steadily and directly at the high pressure limit. This can be the case, for example, when heaving a freight car in shunting operation or when overcoming an obstacle in the field.
In the hydrostatic-mechanical power distribution transmissions existing at present, these uses are not covered due to the high pressure limiting valves which are designed for the permanent load of the hydrostatic transmission. On the other hand, the hydrostatic transmission can work, for a short time, with a definitely higher working pressure. This is under the supposition that the parts can subsequently recover. The torques increases for a short time are no problem for the gearings in the transmission since, at the present time, transmissions are designed based on running smoothly, the certainty of cog failure is generally low.